Periodic dielectric structures, also known as photonic crystals, have the ability of affecting the density of electromagnetic states within their boundaries and even suppressing all modes for a range of frequencies. They can greatly affect the radiative dynamics within the structures and lead to significant changes in the properties of optical devices. This has opened a new and fascinating area for potential applications in optoelectronic devices and has prompted research to find structures that would generate large photonic bandgaps.
Several structures have been found to yield full 3D bandgaps. Examples of these structures are described in the following: Yablonovitch et al., "Photonic Band Structure: The Face-Centered-Cube Case Employing Nonspherical Atoms", Phys. Rev. Lett., Vol. 67, 2295 (1994); Sozuer et al., "Photonic Bands: Simple-Cubic Lattice", J. Opt. Soc. Am. B, Vol. 10, 296 (1993); Ho et al., "Photonic Band Gaps In Three Dimensions: New Layer-By-Layer Periodic Structures", Solid State Comm., Vol. 89, 413 (1994); Sozuer et al., "Photonic Band Calculations for Woodpile Structures", J. Mod. Opt., Vol. 41, 231 (1994); and Ozbay et al., "Micromachined Millimeter-Wave Photonic Band-Gap Crystals", Appl. Phys. Lett., Vol. 64, 2059 (1994); all of which are incorporated herein by reference.
However, conventional fabrication at submicron lengthscales appears to be a difficult endeavor. The only apparent successful microfabrication of a photonic crystal has been reported by Wendt et al., "Nanofabrication of Photonic Lattice Structures in GaAs/AlGaAs", J. Vac. Sci. Tech. B, Vol. 11, 2637 (1993), incorporated herein by reference. The described structure consists of a triangular lattice of cylindrical holes. However, the structure is designed to give rise only to a 2D bandgap.
The primary difficulty with the microfabrication of a 3D photonic crystal comes from the rather sophisticated geometry and intricate arrangement of holes or rods required to open a gap. These complex structures do not easily lend themselves to fabrication at submicron lengthscales. Furthermore, most applications for photonic crystals require bandgaps larger than 10% which in turn requires the use of materials with large index contrasts.